As part of the California Air Resources Board (CARB) On-Board Diagnostics (OBD-II) regulations, the capability for on-board monitoring of a vehicle's pre-catalyst exhaust gas oxygen sensor (O2S) operation must be provided by vehicle manufacturers beginning with the 1994 model year. Typically, the oxygen sensor generates a nearly sinusoidal voltage signal, the amplitude of which can be used as a fingerprint of the sensor operating condition. For example, an attenuated signal can indicate sensor degradation and/or failure.
One technique which complies with the regulations utilizes external air-fuel modulation applied to the engine fuel controller in order to obtain a well-defined signal with which to interrogate the oxygen sensor. Previous implementations of this concept have applied the A/F modulation under openloop conditions. For example, U.S. Pat. No. 5,020,499, issued to Kojima et al., discloses an apparatus for detecting an oxygen sensor abnormality and controlling A/F ratio. Such implementations have experienced difficulties, however, because the mean value of the A/F ratio tends to drift during the test. Although the oxygen sensor switches at the appropriate value, the A/F ratio in the exhaust drifts away from stoichiometry, causing the sensor to react undesirably.
It is, therefore, desirable to ensure that the A/F modulation produces a well-controlled interrogation signal so that the oxygen sensor will react in a well-defined manner, consistent from test to test.